Taxonomic challenges in the identification process, an example - Alder Sawfly, Nematus erythrogaster

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Holden arboretum author- M Onion



The Ohio State Plant and Pest Diagnostic Clinic (PPDC) received a sample of sawfly larvae from Holden Arboretum collected on a species of alder (Alnus spp.) in June 2023. The larvae were eating alder leaves when they were collected by the propagator at Holden Forests and Gardens.


"sawfly on Alnus japonica""Sawfly on Alnus japonica"

Images showing sawfly damage to the leaves. Photo credits – M Onion, Holden Arboretum.


Among the sawflies in North America that are reported to feed on alder, Nematus erythrogaster is the only one that resembles these larvae.


Because of the inability to identify the color-faded larvae due to storage in ethanol, we sent them to the Animal and Plant Health Inspection Services (APHIS) for confirmation. Then, they submitted the sample to the National Identification Services (NIS) for molecular identification.


The morphological analysis resulted in an identification of Nematus sp., but according to molecular analysis, specimens were 99.52% similar to two specimens identified as the European species Pristiphora testacea on BOLD, the Barcode of Life Data System. Reportedly, these specimens in BOLD were collected in Virginia on Alnus glutinosa.


BOLD is a great resource because this database records the genomic sequence that can be used to compare two species to clarify their identity. Simply copy the sequence and paste it into the “Identification Engine” with appropriate request details to compare the similarities of the two species.


While the genus Pristiphora has a wide host range, Pristiphora testacea larvae feed on silver birch and downy birch but have not been reported to feed on alder. Although few Pristiphora larvae are morphologically similar to the larvae collected at Holden Arboretum, P. testacea larvae do not resemble the specimens collected at Holden. P. testacea larvae are green with a distinctive egg-yolk yellow spot on each segment just above the leg. The head is brown in early instars becoming black in mature larvae.


"Pristiphora testacea"

Pristiphora testacea larvae – Although the genus occurs in North America, this particular species is not recorded in North America. Image credit –


Since Pristiphora testacea is a European species, not recorded in the US, and uses different hosts, my conclusion is that the information in BOLD is likely incorrect, the initial identification may be wrong, or the wrong species name was recorded in BOLD by mistake. Upon contacting, they indeed confirmed that the specimens were mislabeled during the routine taxonomic update. There is a high probability that the specimens collected at Holden Arboretum are Nematus erythrogaster.


The Holden Arboretum crew could not collect any adults nor any more larvae last year. The next step is to collect adults this spring from the Holden Arboretum Nursery. They plan to set up some sticky traps around Alnus stock plants and any Alnus found in the surrounding area. When we have adults, we should be able to compare the morphological characteristics and come to a conclusion next summer.